1. Field of the Invention
The present invention relates to a driving circuit for a vibration type actuator used in the driving unit of an image forming apparatus or the like.
2. Related Background Art
A vibration type actuator has, as a basic component, a vibration member formed by bonding a piezoelectric element serving as an electro-mechanical energy conversion element to an elastic member such as a metal member exhibiting small damping of vibrations or a vibration member formed by sandwiching a piezoelectric element between elastic members. A driving circuit applies a driving signal as a cyclic signal to the piezoelectric element to drive the vibration member at a frequency near the resonance frequency, thereby relatively driving the vibration member and a contact member pressed against the vibration member.
According to a vibration member of the above type formed by bonding a piezoelectric element to an elastic member, in general, a plurality of piezoelectric elements are arranged on one surface of, for example, a ring- or disk-like elastic member, and some combinations (to be referred to as phases) of piezoelectric elements are made to obtain a plurality of phases. Driving signals are applied to these phases with temporal phase shifts to form standing waves. By synthesizing these standing waves, a traveling wave is produced in the vibration member, thereby driving a contact member, e.g., a moving member, which is pressed against the surface opposite the surface to which the piezoelectric elements are bonded.
As shown in FIGS. 4, 7, 8, 9, 10, and 11 in Japanese Patent Application Laid-Open No. 8-33364, FIG. 1 in Japanese Patent Application Laid-Open No. 9-271174, and FIG. 3 in Japanese Patent Application Laid-Open No. 11-178364, a conventional driving circuit of a vibration type actuator is comprised of a switching circuit, transformer, pulse generating circuit, and DC power supply. Each circuit is designed to generate a relatively large AC driving signal on the secondary side of the transformer by turning on/off a current supplied from the DC power supply to the transformer at the driving frequency by using the switching circuit.
Such a driving circuit requires switching circuits, transformers, and pulse generating circuits equal in number to the number of phases. For example, for a driving circuit of a two-phase driving vibration type actuator, the circuit shown in FIG. 17 is used. A driving circuit of a four-phase driving vibration type actuator becomes large in scale, as shown in FIG. 18.
Referring to FIGS. 17 and 18, these circuits include transformers 501 to 506, MOSFETs 550 to 573, driving phases 510 to 515 of vibration type actuators, and power supplies 530 and 531. In the circuits shown in FIGS. 17 and 18, pulse generating circuits are not illustrated. In practice, all the gates of the MOSFETs are connected to the pulse generating circuit, and driving signals having a driving frequency are input to switch the direction of a current flowing in the primary coil of the transformer.
Assume that in the driving circuit of the two-phase driving vibration type actuator shown in FIG. 17, the vibration member of the vibration type actuator is formed, for example, by joining a piezoelectric element to one surface of a ring-like elastic member, and a plurality of standing waves having a wavelength xcex are formed in the two phases. In this case, a plurality of regions having different directions of polarization from each other are formed in the ring-like piezoelectric element at intervals of xc2xdxcex (half wavelength), and regions which are polarized in one direction are defined as one phase, such as phase 510. That is, at intervals of xc2xdxcex the phase alternates between one phase 510 and the other phase 511. AC voltages, which are AC signals having a temporal phase shift, are applied to one phase 510 and the other phase 511. Note that in-phase AC voltages are applied to adjacent electrodes, forming polarized regions having different polarities at intervals of a half wavelength, which constitute the respective phases.
A switching circuit for one phase 510 is comprised of the MOSFETs 550 to 553, and a switching circuit for the other phase 511 is comprised of the MOSFETs 554 to 557. The pulse generating circuits (not shown) output pulse signals, each signal having a pulse waveform with a predetermined timing, to the respective MOSFETs in the two switching circuits in accordance with a driving frequency, and AC voltages boosted and wave-shaped by the transformers 501 and 502 are applied to the phases 510 and 511, respectively.
Assume that in the four-phase driving vibration type actuator shown in FIG. 18, the vibration member of the vibration type actuator is formed, for example, by joining a piezoelectric element to one surface of a ring-like elastic member, and a plurality of standing waves having a wavelength xcex are formed in each phase. In this case, regions of the same polarity are formed in the entire circumferential portion of the ring-like piezoelectric element at intervals of xc2xcxcex by using the electrodes. Of the electrodes, alternate electrodes (having intervals of a half wavelength) are defined as one phase, and the remaining alternate electrodes (having intervals of a half wavelength, i.e., having intervals of xc2xcxcex with respect to the first phase) are defined as the other phase. AC voltages having opposite phases are applied to the adjacent electrodes forming one phase. These electrodes are defined as positive phase 512 and negative phase 513. Likewise, AC voltages having opposite phases are applied to the adjacent electrodes forming the respective regions of the other phase. These electrodes are defined as positive phase 514 and negative phase 515.
Switching circuits are comprised of the MOSFETs 558 to 561, MOSFETs 562 to 565, MOSFETs 566 to 569, and MOSFETs 570 to 573. The pulse generating circuit (not shown) outputs pulse signals, each signal having a pulse waveform with a predetermined timing in accordance with a driving frequency, to the respective MOSFETs in the four switching circuits. As a consequence, AC voltages boosted and wave-shaped by the transformers 503, 504, 505, and 506 are applied to the phases 512, 513, 514, and 515, respectively.
A conventional vibration type driving circuit requires a pulse generating circuit, and transformers and switching circuits for the respective phases. This circuit is disadvantageous in terms of cost and space when it is applied to a multi-phase actuator such as a four-phase driving vibration type actuator, in particular.
In addition, as the number of phases increases, the elements constituting a driving circuit vary, resulting in variations in output voltage. This may affect the characteristics of the actuator or its service life.
According to one aspect of the application, it is an object to provide a circuit for driving a four-phase driving vibration type actuator, which can attain a reduction in cost and space saving without increasing the number of parts, and a driving circuit which exhibits a small variation in output voltage between phases without any special adjustment.
According to one aspect of the application, it is an object to provide a driving circuit of a vibration type actuator, which is adjusted to efficiently drive a moving member without increasing the number of parts.
According to one aspect of the application, there is provided a driving apparatus of a four-phase driving vibration type actuator for applying four-phase AC signals having temporal phase differences to four-phase driving phases arranged on an electro-mechanical energy conversion element of a vibration member at intervals of xc2xc a wavelength of a resonance vibration, thereby forming a traveling wave by synthesizing a standing wave formed by one pair of two-phase driving phases at a position of a half wavelength with a standing wave formed by the other pair of two-phase driving phases. The driving apparatus comprises a pulse generating circuit for generating a pulse in accordance with a driving frequency, a first switching circuit for outputting an AC voltage to a first transformer by ON/OFF-controlling a switching element in accordance with a pulse from the pulse generating circuit, and a second switching circuit for outputting an AC voltage to a second transformer by ON/OFF-controlling the switching element in accordance with a pulse from the pulse generating circuit. One pair of two-phase driving phases is connected to two ends of a secondary side of the first transformer, the other pair of two-phase driving phases is connected to two ends of the secondary side of the second transformer, and the pulse generating circuit outputs pulses to the first and second switching circuits with a phase difference of 90xc2x0.
According to one aspect of the application, there is provided a driving apparatus of a vibration type actuator for forming a traveling wave by synthesizing a plurality of standing waves formed by applying AC signals with a temporal phase difference to an electro-mechanical energy conversion element of a vibration member. The driving apparatus comprises two inductors respectively formed on secondary sides of first and second transformers for generating AC voltages, the inductors being set to satisfy
fr less than (fre/2) less than fs
where fre is a parallel resonance frequency of an inductance of the inductors and a capacitance of the electro-mechanical energy conversion element, fr is a resonance frequency of the vibration member, and fs is a frequency at which a vibration amplitude of the vibration member increases for the first time in a region higher than the resonance frequency fr.
According to one aspect of the application, there is provided a driving apparatus of a vibration type actuator for generating a standing wave by applying an AC signal to an electro-mechanical energy conversion element of a vibration member. The driving apparatus comprises a transformer for outputting an AC voltage, and an inductor formed on a secondary side of the transformer, the inductor being set to satisfy
fr less than (fre/2) less than fs
where fre is a parallel resonance frequency of an inductance of the inductor and a capacitance of the electro-mechanical energy conversion element, fr is a resonance frequency of the vibration member, and fs is a frequency at which a vibration amplitude of the vibration member increases for the first time in a region higher than the resonance frequency fr.